Method for the rapid processing of polymer layers in support of imidization processes and fan out wafer level packaging including efficient drying of precursor layers

ABSTRACT

A process for the drying, and subsequent imidization, of polyimide precursors which minimizes or eliminates voids and which minimizes or eliminates discoloration. The process uses a sequential set of descending pressure operations that allow for time efficient processing of wafers. The set of descending pressure operations are interspersed with evacuation processes using heated gasses, which combine heating and byproduct evacuation. The process results in layers with reduced or eliminated voiding, discoloration, and solvent retention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a reissue application of U.S. patent applicationSer. No. 16/174,260, filed Oct. 29, 2018 (now U.S. Pat. No. 10,319,612),is a continuation of U.S. patent application Ser. No. 15/206,318 toMoffat, filed Jul. 11, 2016, which claims priority to U.S. ProvisionalPatent Application No. 62/281,723 to Moffat, filed Jan. 21, 2016, andwhich claims priority to U.S. Provisional Patent Application No.62/303,391 to Moffat, filed Mar. 4, 2016, all of which are herebyincorporated by reference in their entirety.

BACKGROUND Field of the Invention

This invention relates to the coating of substrates, and in particularto a process for more effective and rapid drying of polymer layers.

Description of the Related Art

A continuing trend in semiconductor technology is the formation ofintegrated circuit (IC) chips having more and faster circuits thereon.Such ultralarge scale integration has resulted in a continued shrinkageof feature sizes with the result that a large number of devices are madeavailable on a single chip. With a limited chip surface area, theinterconnect density typically expands above the substrate in amulti-level arrangement and the devices have to be interconnected acrossthese multiple levels. The interconnects must be electrically insulatedfrom each other except where designed to make contact. Usuallyelectrical insulation requires depositing dielectric films onto asurface, for example using a CVD or spinning-on process. The shrinkagein integrated circuit design rules has simultaneously reduced the wiringpitch. These have made the signal propagation delay in the interconnectsan appreciable fraction of the total cycle time. The motivation tominimize signal delay has driven extensive studies to develop a lowdielectric constant (low-k) material that can be used as an inter-leveldielectric in integrated circuit (IC) manufacturing. The majority oflow-k materials used in the ILD layer are based on thermally curedspin-on organic or inorganic polymers.

Polyimide is a polymer material often used in the production ofsemiconductor substrates such as silicon wafers. Polyimide is adesirable insulating material for semiconductor wafers because of itsoutstanding physical properties. Unfortunately, polyimide typicallyrequires a long time to cure when conventional heating techniques areused. A cure cycle of several hours is typical and this often becomesthe pacing step in semiconductor fabrication. In addition, there areother problems involved with curing polyimide resin with conventionalheat. For example, when polyimide resin is cured in a conventionalfurnace, the outer surface of the resin typically cures faster than thecenter portions. This can cause various physical defects, such as theformation of voids, and can result in inferior mechanical propertiessuch as reduced modulus, enhanced swelling, solvent uptake, andcoefficient of thermal expansion.

A polyimide precursor may be applied to a substrate, and then dried toprepare for imidization of the polymer. A goal of the drying process isto remove the solvent from the polymer (which may beN-Methyl-2-pyrrolidone, NMP, for example), and it can also be importantto remove oxygen during the drying process. In addition, further goalsof the drying process are to minimize or eliminate any bubbles/voids inthe polymer layer, to minimize discoloration to the layer that may beinduced by heating, and to fully remove residual solvent from theprecursor mix. Each of these items sought to be eliminated may interferewith subsequent process steps, or enhance the probability of failure ofa device containing the polyimide layer.

In addition to need to dry polyimide precursor layer, some semiconductormanufacturing processes present the need to outgas plastic moldcompounds. For example, in Fan Out Wafer Level Processing (FOWLP), theplastic molding compounded which the semiconductor die are molded intomay require outgassing to remove compounds that will interfere withsubsequent processing, such as metallization steps.

What is called for is a process which can quickly dry a polymercompound, which may be a polyimide precursor. What is further called foris such a process that minimizes or eliminates void, discoloration, andthat also results in thorough solvent removal. What is also called foris a process which can both quickly dry a polyimide precursor and outgasplastic molding in a single step, greatly reducing processing time.

SUMMARY

A process for the drying, and subsequent imidization, of polyimideprecursors which minimizes or eliminates voids and which minimizes oreliminates discoloration. The process uses a sequential set ofdescending pressure operations that allow for time efficient processingof wafers. The set of descending pressure operations are interspersedwith evacuation processes using heated gasses, which combine heating andbyproduct evacuation. The process results in layers with reduced oreliminated voiding, discoloration, and solvent retention. The processmay also be used during Fan Out Wafer Lever Processing fabrication stepsto outgas plastic molding compounds, which may be outgassedsimultaneously during the polyimide precursor drying steps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustrative view of a substrate.

FIG. 2 is an illustrative view of a substrate with a polyimide precursorlayer.

FIG. 3 is an illustrative view of a substrate with a polyimide precursorlayer.

FIG. 4 is an illustrative view of a substrate with a polyimide precursorlayer.

FIG. 5 is an illustrative view of a substrate with a polyimide precursorlayer.

FIG. 6 is an illustrative view of a substrate with a polyimide precursorlayer.

FIG. 7 is an illustrative view of a substrate with a polyimide precursorlayer.

FIG. 8 is an illustrative view of a process oven which may be used inprocesses according to embodiments of the present invention.

FIG. 9 is an illustrative view of die reconfigures for FOWLP processing.

FIG. 10 is an illustrative view of the molding of die.

FIG. 11 is an illustrative side section of a FOWLP configuration.

DETAILED DESCRIPTION

A method for the drying of a polyimide precursor such as a layer on asilicon substrate. The method may be significantly quicker than priordrying methods, while also resulting in polyimide layers with fewer orno defects. In some prior processes, the drying is incomplete as thelayer is not sufficiently dried as the higher temperature imidizationprocess is transitioned into, resulting in bubbling within the layer.The bubbles are voids which can result in faults that cause failures inthe end product, such as microchips, for example. In some devices,multiple layers of polyimide may be used, with the possibility offailure compounded with each additional layer.

In some prior processes, the polyimide precursor layer may becomediscolored due to exposure of the layer to oxygen at temperature. Thelayer may become brown, which will significantly interfere with thetransmission of light through the cured polyimide layer. The layers mayneed to retain their clarity to allow for retention of sight of lowerlevel alignment features or marks. In some aspects the polyimide layermay also be a photosensitive layer, and browning of the layer mayinterfere with the exposure of the layer. In embodiments of the presentinvention, the drying steps of the process not only result in a driedpolyimide precursor layer with the advantages as listed above anddescribed herein, but also result in the process oven having a veryextremely low oxygen level, allowing for further processing of thesubstrates in that same process oven in an extremely low oxygen levelenvironment.

In some aspects, semiconductor die are molded into a plastic moldingcompound in support of Fan Out Wafer Level Packaging (FOWLP)fabrication. After the molding, a number of subsequent process steps mayoccur. Subsequent metallization steps may need to be done at very lowpressures, and outgassing of the plastic molding compound interfereswith this metallization step, by extending the time it takes to get tothe reduced metallization pressure (vacuum level). Typically there maybe a polyimide layer over the plastic molding compound, which is put inplace prior to a metallization step. In some prior processes, anoutgassing step for the plastic molding compound was needed, as well asa drying step for the polyimide precursor prior to imidization. In someembodiments of the present invention, a single process may be used toboth outgas the plastic molding compound and to dry the polyimideprecursor layer. As discussed below, polyimide precursor dryingaccording to embodiments of the present invention provides the unusual,and significantly improved, circumstance of greatly reducing processingtime while also significantly reducing defect levels in the polyimidelayer after imidization, resulting in a higher overall throughput in ashorter time. Coupled with plastic molding compound outgassing duringthe later FOWLP steps, embodiments of the present invention may replacetwo longer process steps with a single shorter step (shorter than eitherof the previously required two steps).

In an exemplary embodiment, a substrate is ready for further processing.In some aspects, the substrate may be a silicon wafer, which may havebeen doped or otherwise processed. FIG. 1 illustrates a substrate with atop surface 101. The substrate 100 may be a silicon substrate. Thesubstrate may be a circular silicon substrate of up to 14 inches indiameter. FIG. 2 illustrates a substrate 100 with a polyimide precursorlayer 102. The polyimide precursor 102 has a top surface 103. A liquidpolyimide precursor is applied over a substrate, or over prior layersalready applied over a substrate. The polyimide precursor may have asolvent such as NMP.

In some aspects, a drying process is implemented to completely or nearlycompletely dry the precursor layer prior to going to a highertemperature for imidization, as with temperature imidization processes.FIG. 3 illustrates a substrate with a polyimide precursor layer 102.With a drying process, the solvent is liberated 104 from the polyimideprecursor layer. At room temperature, NMP boils at approximately 205 C.A drawback with raising the polyimide precursor layer to the NMP boilingtemperature is that the polyimide precursor layer may begin to skin overat 200 C. FIG. 4 illustrates a substrate 100 with a polyimide precursorlayer 102. In this illustrative example of a non-favored result, askinning layer 105 has formed along the top surface 103 of the polyimideprecursor layer 102. A skinning layer 105 may interfere with theliberation of evaporating solvent from the polyimide precursor layer102.

The skinning over of the polyimide precursor layer may trap theevaporating solvent within the polyimide precursor layer, interferingwith liberation of gas from the layer as the solvent evaporates.Retained solvent may then result in large bubbles in the imidized layeras the volume of the solvent as a gas at the higher imidizationtemperatures increases, and then is made permanent as the polyimidelayer becomes hard via imidization. FIG. 5 illustrates a non-favoredcondition wherein bubbles 106 have formed within the polyimide precursorlayer 102, and are being prevented from being liberated 104 from thepolyimide precursor layer by the skinning layer 105.

FIG. 6 illustrates another non-favored condition wherein large bubbles107 are forming within the polyimide precursor layer 102. In some priorprocesses, driving the drying process with too low of a pressure tosoon, as opposed to stepped drying pressures according to someembodiments of the present invention, may result in large bubbleformation. Also, in cases where the polyimide precursor is not fullydried prior to the raising of temperature to imidization temperature,bubbles may form during this temperature rise as residual solventevaporates but is trapped in the already hardening polyimide precursorlayer as imidization is occurring. The resultant bubbles 107 reduce thereliability of the polyimide layer, and may result in faults in thecompleted semiconductor.

Large bubbles may also travel to the surface of the drying polyimideprecursor resulting in polyimide popping, leaving craters in thesurface. FIG. 7 illustrates popped bubbles 108 in a polyimide layer 102on a substrate 100. The bubbles are then voids in a needed protective,insulating, layer which may result in faults in the final productsemiconductor, using one example.

In some embodiments of the present invention, the drying process iscarried out in a process chamber with low pressure/vacuum capabilities.The process chamber may also include capability for inletting heatedinert gas, such as nitrogen. The process chamber may also be able to beheated for supporting the drying process. The process chamber may alsobe able to be heated to even higher temperatures to support temperatureimidization processing after the drying portion of the process.

With reduced pressure, the solvent will boil at a lower temperature. Forexample, NMP boils at approximately 105 C at 50 Torr. Using an exampleof a substrate coated with a polyimide precursor, or a plurality of suchcoated substrates, the substrates are delivered into a process chamber.The process chamber may be heated to a temperature below the roomtemperature boiling point of the solvent. The solvent may be NMP and theinitial heating temperature may be 150 C. The pressure used is subjectto at least two conflicting constraints. On the one hand, the pressureshould be reduced enough to evaporate the solvent, allowing for the lowpressure liberation of the gas which permeates the liquid/gel precursorand is liberated to the low pressure chamber. On the other hand, toomuch evaporation, too quickly, could lead to aggregation of the gas intobubbles, which may lead to popping on the surface or other issues.Further, though, lowering the chamber pressure in further steps to apressure even lower than 50 Torr creates more pressure differentialbetween the bottom of the gel, against the substrate, and the lowpressure chamber, better driving out the gas. These goals and risks arewhat are now addressed below.

In an exemplary process according to some embodiments of the presentinvention, a polyimide precursor is applied to a silicon substrate. Insome aspects, the polyimide precursor is applied directly over thesilicon substrate. In some aspects, the polyimide precursor is appliedover other layers already on a substrate, which may be other polyimidelayers and metal layers, for example. In some aspects, the solvent usedin the polyimide precursor is NMP. An expected thickness forsemiconductor applications is in the range of 7-10 microns. Although asingle substrate could be processed, in some aspects a plurality ofsubstrates may be processed. As seen in FIG. 8 , a process oven 201 maybe used to support a plurality of substrates 203 within a chamber 202.The process oven may include internal heaters, heated inert gas inputs,and vacuum capability. The substrates are placed into the chamber 202that has been heated to 150 C. In some aspects, the chamber is heated toa temperature in the range of 135 C to 180 C. The chamber pressure isreduced to a first drying pressure of 50 Torr. In some embodiments, thefirst drying pressure is in the range of 30-60 Torr. After reaching thefirst drying pressure, the chamber may then be flushed with a heatedinert gas such as nitrogen at a pressure of 600 Torr. In some aspectsthe heated inert gas may be at a pressure in the range of 550 to 760Torr. The nitrogen may be heated to the same temperature as the chamber,150 C. The chamber pressure is then reduced to a second drying pressureof 25 Torr. In some embodiments, the second drying pressure is in therange of 15-30 Torr. After reaching the second drying pressure, thechamber may then be flushed with a heated inert gas such as nitrogen ata pressure of 600 Torr. In some aspects the heated inert gas may be at apressure in the range of 550 to 760 Torr. The nitrogen may be heated tothe same temperature as the chamber, 150 C. The chamber pressure is thenreduced to a third drying temperature of 1 Torr. In some embodiments,the third drying pressure is in the range of 1-15 Torr. After reachingthe third drying pressure, the chamber may then be filled with heatedinert gas, such as nitrogen, up to 650 Torr, in preparation forimidization of the polyimide precursor. The substrates may then undergotemperature imidization in the same chamber. As described further below,the oxygen level in the process oven may now be very extremely low. Thesubsequent temperature imidization may occur at 350-375 C, and asfurther described below.

In an exemplary embodiment further illustrating the timing of a processas described above, a process may begin with the heating of the processoven to a temperature of 150 C. A single substrate or a plurality ofsubstrates within the process oven, which include a polyimide precursorincluding a solvent such as NMP, are put into the process oven which hasbeen preheated to the temperature of 150 C. The process oven pressure isthen reduced to a first drying pressure of 50 Torr. This portion of theprocess may take 2-3 minutes. The process oven is then flushed withpreheated nitrogen heated to 150 C up to a pressure of 600 Torr. Thisportion of the process may take 2-3 minutes. The process oven pressureis then reduced to a second drying pressure of 25 Torr. This portion ofthe process may take 3-4 minutes. The process oven is then flushed withpreheated nitrogen heated to 150 C up to a pressure of 600 Torr. Thisportion of the process may take 2-3 minutes. The process oven pressureis then reduced to a third drying pressure of 1 Torr. This portion ofthe process may take 4-5 minutes. The process oven is then flushed withpreheated nitrogen heated to 150 C up to a pressure of 650 Torr. Thisportion of the process may take 2-3 minutes. The aforementioned stepshave now greatly reduced the oxygen level in the process oven, as wellas having removed all or nearly all of the solvent from the polyimideprecursor with little or no bubbling or skinning of the polyimideprecursor. The aforementioned steps combine the above mentionedadvantages of reduced oxygen levels in the process oven and a precursorlayer with little or no bubbling or skinning, while being performed morequickly than other currently used processes. The other currently usedprocesses not only take longer, but they also result in polyimideprecursor layers which have bubbling, skinning, popping, or otherdetrimental attributes, and further, do not have the sought after verylow oxygen level.

After the multi-step drying process, the substrates are now ready fortemperature imidization. As discussed further below, the oxygen level inthe process oven may now be down as low as approximately 1 ppm, as anend result of the drying process. An exemplary temperature imidizationprocess may now include maintaining approximately 250 Torr in theprocess chamber while inputting heated nitrogen at the top of theprocess oven while pulling vacuum at the bottom of the process oven. Theheated nitrogen and the oven temperatures may now be raised in unison,for example, to 350 C. At 4 C/minute, this heating process would take 50minutes. At 350 C the oven and gas temperatures may be held for 1 hourfor temperature imidization of the polyimide precursor. Although 350 Cis an illustrative temperature using NMP, other temperatures may usedfor the temperature imidization. After the temperature imidization, theoven heaters may be turned off, which will result in a cooling of theoven. The heated nitrogen flow may be cooled at a rate which tracks thecooling oven.

The vacuum pulsing as described above, in conjunction with theintervening flushing with nitrogen, provides another important benefitfor the imidization process, which is already separately enhanced by thesignificantly enhanced drying. The vacuum pulsing and interveningflushing results in a much lower oxygen level in the process chamber asthe substrates go further in the process. In contrast to prior methodswhich pull vacuum once for a combined drying/imidization process, thevacuum pulsing reduces the partial pressure of oxygen significantly, aseach flushing with nitrogen resets the initial gas balance prior to thenext pull of vacuum. The inflows of heated inert gas enhance the heatingof the polyimide precursor layer, as well as the fixturing within thechamber and the chamber itself. The reduced pressure of the processdescribed herein enhances, and speeds up, the evaporation of thesolvent, and also allows for a temperature to be used for theevaporation that is below the skinning temperature of the polyimideprecursor. The staging of the reduced pressure at sequentially lowerpressures reduced bubbling which might occur by simply going straight toa very reduced pressure, and avoids the residual solvent which wouldremain if the very reduced pressure is not utilized. Residual solventmay inhibit the imidization of the polyimide precursor. The pulsing andflushing then further adds the benefit of a final chamber compositionwith significantly less oxygen than prior methods, reducing oreliminating the browning which may occur in the polyimide layer duringtemperature imidization in the presence of oxygen.

With the tiered vacuum application, in conjunction with nitrogenflushing, oxygen levels may be driven very, very low. With theabove-described process, the starting concentration of oxygen at theinitial atmospheric conditions is approximately 230,000 parts permillion (ppm). When the initial vacuum is applied down to 50 Torr, andthen the chamber is refilled with nitrogen, the concentration of oxygenwill have been reduced to approximately 15, 131 ppm. With the subsequentvacuum pull down to 25 Torr, and then refilling with nitrogen, theconcentration of oxygen will have been reduced to approximately 498 ppm.With the final vacuum pull down to 1 Torr, and subsequent refilling withnitrogen, the resulting concentration of oxygen will be down to 0.65ppm. The very, very, low oxygen concentrations that result allow for asubsequent processing, including temperature imidization, at oxygenconcentrations well below any prior process. In actual practice, thepurity of the nitrogen supply may become the active parameter in how lowof an oxygen concentration can be reached. If the nitrogen supply isknown to have 10 ppm oxygen, for example, then that will limit the depthof oxygen removal. Some processing chambers may have nitrogenavailability with down to 1 ppm O2, and with such a system oxygenconcentration can be driven down to approximately 1 ppm. Simple flushingof a chamber with pure nitrogen provides some reduction in oxygenconcentration, but such processes are very time consuming and do notachieve results at all comparable to the above-described process.

Further, the use of heated nitrogen for the re-filling steps in theabove-described process works to minimize the effects of freezing thanmay have happened during the vacuum pull. As water boils at 72 C at 50Torr, 26 C at 25 Torr, and −21 C at 1 Torr, the heated nitrogen inputthus facilitated evaporation of any water than may be found in thedevices being dried or outgassed.

In addition to the reduction or elimination of browning, and thereduction or elimination of bubbling and popping in the polyimideprecursor layer, the process as described herein adds anothersignificant improvement that in spite of the much higher qualitypolyimide layer that results, the process is much quicker than priorprocesses. Using a process according to embodiments of this invention, asubstrate, or a plurality of substrates, may be dried and temperatureimidized in 3-3.5 hrs, whereas typically current processes may take upto 12 hours to dry and imidize a polyimide layer. With the increasingnumber of layers in modern semiconductor devices, which may include upto 17 polyimide layers, for example, the time and thus cost savings canbe extremely significant.

In some embodiments, the temperature of the oven during drying is 150 C.In some embodiments, the temperature of the oven is in the range of135-180 C. In some embodiments, the first drying pressure is 50 Torr. Insome embodiments, the first drying pressure is in the range of 30-60Torr. In some embodiments, the second drying pressure is 25 Torr. Insome embodiments, the second drying pressure is in the range of 15-30Torr. In some embodiments, the third drying pressure is 1 Torr. In someembodiments, the third drying pressure is in the range of 1-15 Torr.

In a further embodiment of the present invention, the process used todry the polyimide precursor prior to temperature imidization may also beused to outgass plastic molding compound which may surround asemiconductor die in FOWLP fabrications. Initially, the rapid dryingprocess of polyimide precursor layers may be done to support processingsteps on the entire wafer. Each of the rapid drying steps may bothreduce the time of the drying step relative to prior processes, and alsoincrease reliability of the processed items. With each polyimide layeron the wafer buildup, processes according embodiments of the presentinvention present at least the advantages of reduced processing time,enhanced quality of the polyimide layers, and reduces oxygen levelsafter the drying step.

Once the wafer is completed, the wafer may be sliced up into individualdie. As seen in FIG. 9 , the die may be reconfigured to allow forsubsequent molding steps. In some aspects, these die are then partiallysurrounded by a plastic molding compound, which may allow for fan outconfigurations. FIG. 10 illustrates an exemplary molding process. Thispost wafer, die level, processing may include adding polyimide layers,especially to allow for metallization of leads supporting the fan outconfiguration. FIG. 11 illustrates an exemplary completed fan out waferlevel packaging configuration in cross-section.

The plastic molding compound must be outgassed prior to themetallization process, and this step takes significant time. Further,the polyimide precursor layers should be dried prior to the temperatureimidization process. In some embodiments of the present invention, asingle process may be used to both outgas the plastic molding compoundand to dry the polyimide precursor layer, as opposed to differentprocesses of prior methods. Using the above-described polyimideprecursor drying method will also outgas the plastic molding compound.

In an exemplary embodiment, plastic molding compound is molded around adie. A polyimide precursor layer is applied over a surface which maycomprise both the molding compound and the die. Prior to imidization ofthe polyimide precursor layer, the assembly is dried according to dryingprocesses of the present invention, as described above. This quickprocess will both outgas the plastic molding compound, and dry thepolyimide precursor layer, substituting one process for what hadpreviously been two processes, and which may be of significantly shorterduration.

The applicant builds cure ovens that are specifically designed toaddress the concerns of manufacturers of the cured characteristics ofthe multiple polyimide layers incorporated in Wafer-level Packaging(WLP)/Redistribution Layers (RDL) circuits. Polyimides are hightemperature engineering polymers with excellent mechanical, thermal andelectrical properties. The most important step of the process is thecuring of the polyimide precursors, which can be done under atmosphericor vacuum process conditions. As discussed above, the objectives of aproper cure process are to complete the imidization process, optimizefilm adhesion performance, remove all residual solvents and extraneousgases, and remove photosensitive components.

To convert the polyimide precursors to a stable polyimide film, anelevated temperature (˜250 C to 450 C) extended bake is required forcomplete imidization; it also drives off the N-methylpyrrolidone (NMP)casting solvents and orients the polymer chains for optimal electricaland mechanical properties.

The imidization rate of the polyimide precursors need to be controlledto take into account the differences in thermal expansion coefficientbetween the polyimide film and the underlying substrate. If theimidization rate is not controlled properly, there can be localizedmechanical stress variations across the wafer. In addition, if thecasting solvents evolve non-uniformly across the wafer, film thicknessnon-uniformity can occur due to uneven imidization. The mechanicalstress variations can be observed as wrinkled polyimide film or asdistorted metal lines in the structures under the polyimide layer. Thepolyimide film can also delaminate because film adhesion performance hasnot been optimized. Because mechanical stress variations can affect theyield and reliability of the process, it is critical that controlledtemperature ramp rates are used to provide a larger process window forthe proper curing of a polyimide film.

Non-uniform heating can cause a skin to form on the surface of thepolyimide film during the curing process. The skin can prevent theefficient evolution of the casting solvents and other volatile gases. Ifa cured polyimide film still has residual solvents or other volatilegases, then localized areas of the polyimide film can rupture in aphenomenon known as “popcorning”. These ruptures occur in subsequentprocess steps in tools, which have either a high vacuum or a hightemperature environment. This rupturing is due to the sudden release ofgas bubbles/solvents trapped in the polyimide film that is not properlycured. In addition, a “solvent-free” polyimide film will minimize thequeue time needed to allow for outgassing when the next process step isa high vacuum process, such as metallization.

Photosensitive polyimides offer the advantage of simpler processing byeliminating the need for photoresist compared to standardnon-photosensitive polyimides. This reduces the number of process steps.The curing process parameters, such as temperature, vary with the typeof photosensitive precursors in the polyimide film. For some types ofprecursors, the photosensitive components can be difficult to evolvefrom the polyimide film. Residual photosensitive polyimide precursorscan cause greater internal film-induced stress than those in a standardpolyimide film.

Some photosensitive polyimide precursors and their byproducts also havea tendency to form depositions on the process chamber walls. Heavydeposits can be difficult to remove if the byproducts are notefficiently removed from the chamber during the curing process.Furthermore, when these byproducts are exhausted from the chamber, theyalso need to be substantially removed from the exhaust stream as thebyproducts can redeposit along the exhaust lines. In summary, thephotosensitive components must be eliminated from the polyimide film andefficiently removed from the process chamber.

The presence of oxygen in the process chamber inhibits the propercrosslinking of the polyimide precursors to polyimide thin film. Theresult is incomplete imidization which leads to a brittle film andvariable stress in the polyimide film on the substrate. Also, ambientoxygen darkens the polyimide film. This film transparency is criticalwhen multiple polyimide layers are used during subsequent processing.For multi-layer processes, the alignment marks for the process sequencecan be obscured by the layers of low transparency polyimide films. Insummary, pure nitrogen ambient is required to reduce the level of oxygenin the process chamber.

As described above, embodiments of the present invention allow for morecomplete drying of polyimide precursor layers, more quickly, and withfewer defects, while also offering the additional advantage of theresulting dried substrates/layers residing in an extremely low oxygenlevel environment. The subsequent temperature imidization is thenfurther enhanced in quality. In the case of wafer die with polyimideprecursor layers and molding compound, the outgassing of the moldingcompound may occurs simultaneously with the polyimide precursor drying,further reducing the time needed for such a process, and with higherthroughput.

As evident from the above description, a wide variety of embodiments maybe configured from the description given herein and additionaladvantages and modifications will readily occur to those skilled in theart. The invention in its broader aspects is, therefore, not limited tothe specific details, representative apparatus and illustrative examplesshown and described. Accordingly, departures from such details may bemade without departing from the spirit or scope of the applicant'sgeneral invention.

I claim:
 1. A process for drying of substrates in support of temperatureimidization of polymer layers, said process comprising the steps of:inserting one or more substrates into a process chamber, said one ormore substrates coated with a layer of a polyimide precursor, saidpolyimide precursor comprising N-Methyl-2-pyrrolidone; reducing thepressure in said process chamber to a first drying pressure; first,flushing said process chamber with inert gas to a first flushingpressure; reducing the pressure in said process chamber to a seconddrying pressure after said first flushing, wherein said second dryingpressure is lower than said first drying pressure; second, flushing saidprocess chamber with inert gas to a second flushing pressure after thestep of reducing the pressure in said process chamber to a second dryingpressure; reducing the pressure in said process chamber to a thirddrying pressure after the step of second flushing said process chamber,wherein said third drying pressure is lower than said second dryingpressure; and third, flushing said process chamber with inert gas to athird flushing pressure after the step of reducing the pressure in saidprocess chamber to a third drying pressure.
 2. The process of claim 1wherein said process chamber is heated to a first chamber temperature.3. The process of claim 2 wherein said first flushing said processchamber with inert gas comprises flushing with inert gas heated to afirst gas temperature.
 4. The process of claim 3 wherein said secondflushing said process chamber with inert gas comprises flushing withinert gas heated to a second gas temperature.
 5. The process of claim 4wherein said third flushing said process chamber with inert gascomprises flushing with inert gas heated to a third gas temperature. 6.The process of claim 5 wherein said first chamber temperature is in therange of 135 C to 180 C.
 7. The process of claim 6 wherein said firstdrying pressure is in the range of 30-60 Torr.
 8. The process of claim 6wherein said first drying pressure is in the range of 30-50 Torr.
 9. Theprocess of claim 8 wherein said second drying pressure is in the rangeof 15-30 Torr.
 10. The process of claim 9 wherein said third dryingpressure is in the range of 1-15 Torr.
 11. The process of claim 1 6wherein said first gas temperature is in the range of 135 C-180 C. 12.The process of claim 11 wherein said first gas temperature is and saidfirst chamber temperature have a temperature value in the range of 135C-180 C.
 13. The process of claim 11 wherein said second gas temperatureis in the range of 135 C-180 C.
 14. The process of claim 12 13 whereinsaid first chamber temperature, said first gas temperature, and saidsecond gas temperature is have a temperature value in the range of 135C-180 C.
 15. The process of claim 13 wherein said second third gastemperature is in the range of 135 C-180 C.
 16. The process of claim 1415 wherein said first chamber temperature, said first gas temperature,said second gas temperature is, and said third gas temperature have atemperature value in the range of 135 C-180 C.
 17. The process of claim16 1 further comprising the step of temperature imidizing a polymerlayer on said one or more substrates the layer of polyimide precursorafter the step of third flushing said process chamberwhile havingmaintained the low oxygen level of the drying steps.
 18. The process ofclaim 17, wherein the step of temperature imidizing occurs at atemperature between 350-375 C.
 19. The process of claim 1, wherein saidfirst flushing pressure, said second flushing pressure, and said thirdflushing pressure have a same pressure value.
 20. The process of claim19, wherein the same pressure value is between 550 to 760 Torr.
 21. Amethod for curing polymer coated substrates, comprising: (a) placing oneor more substrates including a polymer layer in a process chamber of aprocess oven; (b) reducing the pressure in the process chamber to afirst drying pressure; (c) after step (b), flushing the process chamberwith heated inert gas; (d) after step (c), reducing the pressure in theprocess chamber to a second drying pressure lower than the first dryingpressure; (e) after step (d), flushing the process chamber with heatedinert gas; and (f) after step (e), temperature imidizing the polymerlayer of the one or more substrates.
 22. The method of claim 21, furtherincluding, after step (e) and before step (f), reducing the pressure inthe process chamber to a third drying pressure lower than the seconddrying pressure.
 23. The method of claim 22, further including, afterreducing the pressure in the process chamber to the third dryingpressure and before step (f), flushing the process chamber with heatedinert gas.
 24. The method of claim 22, wherein the first drying pressureis between 30-60 Torr, the second drying pressure is between 15-30 Torr,and the third drying pressure is between 1-15 Torr.
 25. The method ofclaim 21, wherein the first drying pressure is between 30-60 Torr andthe second drying pressure is between 15-30 Torr.
 26. The method ofclaim 21, wherein, before step (b), heating the process chamber to atemperature between 135-180 C.
 27. The method of claim 26, whereinflushing the process chamber in steps (c) and (e) includes flushing theprocess chamber with heated inert gas to a pressure in the range of550-760 Torr.
 28. The method of claim 27, wherein flushing the processchamber in steps (c) and (e) includes flushing the process chamber withinert gas heated to a same temperature as the process chamber.
 29. Themethod of claim 21, wherein the inert gas is nitrogen.
 30. The method ofclaim 21, wherein temperature imidizing the polymer layer includesheating the process chamber to an imidization temperature between350-375 C, and maintaining the process chamber at the imidizationtemperature for an hour.
 31. A method for curing polymer coatedsubstrates, comprising: (a) heating the process chamber of a processoven to a first temperature; (b) after step (a), placing one or moresubstrates including a polymer layer in the heated process chamber; (c)after step (b), reducing the pressure in the process chamber to apressure between 30-60 Torr; (d) after step (c), flushing the processchamber with heated inert gas at the first temperature to increase thepressure in the process chamber to a value between 550-760 Torr; (e)after step (d), reducing the pressure in the process chamber to apressure between 15-30 Torr; (f) after step (e), flushing the processchamber with heated inert gas at the first temperature to increase thepressure in the process chamber to a value between 550-760 Torr; (g)after step (f), temperature imidizing the polymer layer of the one ormore substrates.
 32. The method of claim 31, wherein the firsttemperature is between 135-180 C.